1. Field of the Invention
This invention relates to a method of detecting sector servo information recorded on a disk surface in a recording and reproducing system. In providing servo information in a disk system, two methods are generally utilized. In a first method, the servo information is recorded on a separate disk or on a separate disk surface and, in a second method, it is recorded interspersed on the same disk surface as the work data. The present invention particularly relates to the second method, in which the work data and the servo information are arranged in alternating sectors on the same disk surface. In the present invention, this method is conventionally called sector servo method in a disk system.
2. Description of the Related Art
FIG. 1(a) is a schematic and block diagram of a disk system for explaining the general principle of the sector servo method. The disk system comprises a disk enclosure DE and control circuit CT connected to a higher command system.
The disk enclosure DE comprises a spindle motor 11 which drives a plurality of disks 10, an actuator 12 which is driven by a voice coil motor (VCM), a plurality of magnetic heads 13 fixed to the actuator 12, and a pre-amplifier 14. The magnetic heads 13 are disposed on both surfaces of each disk 10 and can move within a specified zone on the disk surface with the forward and backward movements of the actuator 12. The pre-amplifier 14 is provided to adjust signal levels of an output of the magnetic head 13 and is input to a main amplifier (not shown) during read and write periods of the disk system.
The control circuit CT has functions of controlling such as rotational movement of the spindle motor 11, radial positioning of the magnetic head 13 on the disk surface, and controlling read/write operations of the magnetic head, detection of the sector servo information and interface operation with the higher command system.
One surface of the disk 10 is divided into a plurality of sectors, which are shown schematically in FIG. 1(b). In the Figure, eight servo information sectors 10b (briefly called servo sector) are interspersed alternately with eight work data sectors 10a (briefly called data sector). Sector servo information includes track address information and servo position information, and further includes supplementary code information such as a code sequence for automatic gain control for the associated circuits and a code sequence for discriminating the servo sector from the data sector.
When the disk system receives a command to access the specified track number on the disk surface, the magnetic head is made to move within the immediate proximity of the target track. After detecting the servo position information of the sector servo information, the magnetic head 13 is controlled to be positioned in accurate alignment with the target track.
In order to read the sector servo information, this servo information should be discriminated at first from the work data among a reproduced signal sensed by the magnetic head. A discrimination method utilizing dependency on the angular position of the spindle motor 11 is apt to become inaccurate due to a change of the rotation of the spindle motor, a temperature change in the disk enclosure DE, etc. Therefore, the following method of discrimination has been used in the prior art.
FIG. 2 shows schematically a portion of a plurality of tracks in one of servo sectors, wherein the sector servo information is divided into plural zones. Only four tracks n-1, n, n+1, and n+b 2 are illustrated in the FIG. 2, in which the downward direction shows an inward radial direction of the disk surface. The servo sector 10b comprises a plurality of zones such as preamble zone, sector mark zone, gray code zone designating a track address, AGC zone, servo position A and B zones and postamble zone. Among these zones, preamble and postamble zones are provided to separate the sector servo information from the work data information. The sector mark is utilized as an identification code for beginning of the sector servo information.
An example of an output signal of the sector servo information is schematically illustrated by the waveform of FIG. 3(a). The output of the sector mark is a DC erase pattern of three byte length. The sector mark output is followed by a track address information encoded in the known Gray code sequence. And next, an AGC (Automatic Gain Control) signal output follows, which is a regular sequence of codes such as 1, 1, . . . 1, and is used to fix signal levels in the associated circuits at a predetermined level. Finally, servo position A and B outputs follow thereafter.
As shown in FIG. 2, each of servo position information is recorded in a track having a half track pitch deviation from the track for other information. When the magnetic head travels in accurate alignment with the target track, the detected servo position outputs A and B have substantially the same level. However, when the magnetic head position deviates from the center line of track, the detected servo position outputs A and B have different signal levels, which are sensed and used for aligning the magnetic head.
In order to extract each information included in the sector servo information from the output signal from the magnetic head, it is very important problem to generate a plurality of timing signals which are synchronous with the selected portion of the sector servo information. For generating the timing signals, a counting circuit means, which is comprised in a demodulation circuit of sector servo information, is utilized and is set to start at an appropriate time t as shown in FIG. 3(d). The counting circuit means generates a specified signal, so called, a sector window signal as shown in FIG. 3(b), during which a sector mark detection signal SM is generated as shown in FIG. 3(c). The sector mark detection signal further triggers the above counter circuit means at time t and thus it makes the counter circuit means synchronous with the output signal of the sector servo information.
Because the sector mark signal of three byte DC erase pattern should be detected during the sector window signal of FIG. 3(b), the sector window signal should be made synchronous with the sector servo information.
In the sector servo method of the prior art, the synchronization is obtained solely depending on detection of the sector mark signal composed of the DC erase pattern of three byte length. Though the DC erase pattern of three byte length is quite different from other series of code patterns such as the work data recorded on disk surface, the detection of the sector mark signal causes a problem when the sector mark comprises defects in the recording medium or the detection thereof is affected by outward noise pulses.
If the sector mark signal can not be detected, then the sector servo information can not be detected and further, in some cases, the recorded sector servo information will be destroyed during write operation of the disk system. Moreover, when the sector mark signal can not be detected, the sector window signal goes out of synchronous condition, which further increases a difficulty in detecting the sector mark.
To solve the above problem, the following references are disclosed in the Japanese Unexamined Patent Publications.
SH0-59-36374 dated Feb. 28, 1984 by M. Tsunekawa et al.
SH0-63-142576 dated June 14, 1988 by K. Yamada.